Half Marathon Training Plan: A Realistic 12-Week Build to 13.1
Twelve weeks is the sweet spot for a half marathon build — long enough to gain real fitness, short enough that life doesn't derail it. Here's the structure that produces PRs without burnout.
Twelve weeks is the sweet spot for a half marathon. Long enough to build genuine fitness, short enough that adherence stays high. This is the structure I use with the majority of intermediate athletes coming off a base of 20–25 mpw and chasing a PR.
Weekly structure
- Monday — full rest or 30' cross-train
- Tuesday — quality session (intervals or threshold)
- Wednesday — easy 40–50' + strides
- Thursday — tempo or progression run
- Friday — easy 30–40' or rest
- Saturday — easy run + strides
- Sunday — long run (with race-pace work in middle phase)
Phase 1 — Aerobic build (weeks 1–4)
Mileage ramps from 25 to 35 mpw. Long runs grow from 8 to 12 miles, all conversational. Tuesdays: 6×600m at 5K pace. Thursdays: 20-minute tempo at half-marathon pace + 5 minutes. The job here is durability — build the chassis before adding speed.
Phase 2 — Specific (weeks 5–9)
Mileage peaks at 40 mpw. Long runs hit 13–15 miles, with 6–8 miles at goal half-marathon pace in the back half. Tuesdays move to longer threshold reps: 4×1mi at half-marathon pace + 5 seconds. This is where PRs are forged.
Phase 3 — Sharpen and taper (weeks 10–12)
Mileage drops 20% in week 11, 40% in race week. Intensity stays — short, fast efforts to preserve sharpness. Final long run 10 days out: 10 miles with 4 at goal pace. Race week: easy short runs, strides every other day, full carb load 36 hours out.
The half marathon punishes the under-trained worse than the over-trained. Build slowly, race aggressively.
Pace targets by goal time
- Sub-2:00 — 9:09/mile, 25-35 mpw, long run 12mi
- Sub-1:45 — 8:00/mile, 35-45 mpw, long run 14mi
- Sub-1:30 — 6:52/mile, 45-55 mpw, long run 14-16mi
- Sub-1:20 — 6:06/mile, 55-65 mpw, long run 16-18mi
Common 12-week mistakes
- 01Adding mileage faster than 10% per week
- 02Skipping the easy days because you 'feel good' — easy days build the engine
- 03Racing other distances mid-cycle (a hard 5K 4 weeks out is a 5-day setback)
- 04Tapering too aggressively — sluggish legs on race day mean too much rest, not too little
Questions athletes ask about this
- Can a beginner train for a half marathon in 12 weeks?
- If you can comfortably run 4–5 miles continuously, yes. Pure beginners should plan 16–20 weeks to avoid injury.
- How many miles per week do I need for a sub-2:00 half?
- 25–35 mpw consistently, with a 12-mile long run. Anything less is doable but increases the risk of fading in the second half.
- What pace should my long runs be?
- 60–90 seconds per mile slower than goal HM pace. The job of the long run is time on feet, not speed.
- Should I race a 10K during the build?
- One tune-up 10K around weeks 8–9 is helpful — fitness check and pacing practice. Don't race more than once mid-cycle.
- How long should the half-marathon taper be?
- 10 days for most athletes. Drop volume 20% then 40%, keep intensity, finish with short race-pace touches in race week.
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Book a discovery callEndurance coach since 2015. RRCA-certified, USAT Level II, TrainingPeaks Level 2. 12× Boston Marathon qualifier. Based in Fenway, Boston — coaching athletes worldwide in English, Portuguese, and Spanish.
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